Suction pumps are used in many processes to transfer liquids from first locations to second locations. A typical suction pump includes a pump casing that defines a pump chamber. A rotor that is rotated by a motor is seated in the pump chamber. Seated in the rotor are a number of vanes. As the rotor turns, the centrifugal force developed urges the vanes outwardly towards the wall of the pump chamber that defines the pump chamber. Owing to the geometry of the pump chamber and the position of the rotor in the pump chamber, a vacuum develops in the interstitial spaces between the vanes, referred to as fluid cavities. As each fluid cavity is presented to the inlet of the pump chamber, this vacuum presents a suction head to the liquid being pumped. Liquid is thus drawn into the fluid cavity and rotates with the fluid cavity. As the rotor turns, the size of the fluid cavity decreases as it approaches the outlet of the pump chamber. This change in size of the fluid cavity forces the liquid out of the pump chamber and through the outlet line connected to the pump.
One particular fluid delivery system in which a suction pump is often employed is a fuel dispensing system. A typical fuel dispensing system is designed to draw fuel from an underground storage tank in which the fuel, (gasoline, diesel fuel, kerosene, alcohol, liquid-state propane, liquid-state butane, other liquified gases and other liquid-state fuels that are highly volatile) is stored. The dispensing system includes a pump that forces the fuel to and through an above ground hose-and-nozzle subassembly. Flow through the pump is often regulated by a nozzle-controlled on/off valve. There is also a flow meter that monitors the volume of fuel dispensed to provide the data required to ensure that the customer is accurately charged for the quantity of fuel delivered. When a suction pump is employed in a fuel dispensing system, the pump draws fuel from the storage tank and then forces it through the downline dispensing system components. Should any leaks develop in the supply line from the storage tank, the suction drawn by the pump, instead of allowing the fuel to flow out, will draw air into the line. Thus, employing a suction pump in a fuel dispensing system serves to minimize unwanted fuel leakage and the attendant environmental damage such leakage can foster.
While suction pumps serve as useful devices for generating a fluid flow in many systems, such as fuel dispensing systems, there are some disadvantages associated with their use. One particular disadvantage associated with many fluid pumps is that when they are running, they generate a significant amount of noise. This noise is generated because, as the liquid enters a fluid cavity, it has an opportunity to expand volumetrically. Some liquids partially vaporize. Then, when the fluid cavity decreases in size, the liquid compresses. This compression causes the bubbles of vaporized fluid to collapse. This collapsing, "popping," of the vapor bubbles can generate significant amounts of noise. This vaporization and subsequent condensation of fluid is especially prone to occur if the fluid is volatile when in the liquid-state, as are many fuels. In a fuel dispensing system, the suction pump is typically located in the above-ground housing that contains most of the other components of the dispensing system. Thus, the noise generated by the pump during its operation can readily be heard by an individual using the dispensing system. If the fuel has a relatively high vapor pressure, the noise can be relatively loud. If the noise is loud enough, the person using the system may even become so concerned that he/she will stop pumping fuel due to a belief that the dispensing system is malfunctioning. Once a person takes this step, it appreciably lengthens the overall time it takes to perform the fuel dispensing process.
There have been some attempts to minimize the of noise generated by suction pumps by slowing the r of rotation of the pump rotor. A disadvantage of this technique is that, for a given size pump, it reduces the rate at which the pump pumps liquid. Consequently, in some liquid dispensing systems, it is necessary to increase pump size in order to compensate for this drop in liquid-pumping efficiency. Other attempts have been made to reduce the amount of noise that is generated by simply providing acoustic insulation around the pump. This insulation serves to increase the overall size of the pump. These larger pumps required to hold the generated noise to a minimum can be(difficult to install in locations where space is at a premium, such as the inside of a fuel dispenser housing.